ELECTRONICS

Electronics goes beyond simple electricity. Using the TRANSISTOR and other COMPONENTS, such as resistors and
capacitors, electronics allows us to control large electric currents with small
electric currents. This opens up a whole new world. Electronics can amplify
sound, make radio waves, or handle computer data. OPTOELECTRONICS
can use light to work a remote control or send messages across the
globe.

TRANSISTOR TEAM

In 1947, at the Bell Telephone Laboratories in the USA, John Bardeen
(1908–1991, left), Walter Brattain (1902–1987, right), and
William Shockley (1910–1989, centre) invented a small, solid device that
could amplify electrical signals. They called it a transistor. Until then, the
only practical amplifiers were based on fragile glass tubes with a vacuum
inside. The team won the Nobel Prize for Physics in 1956.

CIRCUIT BOARD

Electronic circuits are built by fixing components into a plastic
board that has copper tracks on one side to link them together. The components
are secured and connected by melting a metal called solder around their pins.
Modern boards may have several layers of tracks. Some boards contain only part
of a circuit. These plug into a mother board, which links several daughter
boards to form a complete circuit.

COMPONENTS

Components are parts from which electronic circuits are
built. Each component responds to electricity in a particular way. For example,
capacitors block steady currents, while resistors let them through. By
connecting up the right components, engineers can build anything from a door
chime to a computer.

Capacitors store electric charge. They contain two sets of
insulated metal plates and can carry signals between two points without letting
direct current through. Electrolytic capacitors hold more charge than other
types but they need a steady voltage across them to work.

RESISTOR

Resistors control currents and voltages. The current through a
resistor is given by the voltage across it divided by its resistance. This
means that a resistor can convert a voltage into the corresponding current. On
the other hand, if a current is passed through a resistor, it can produce the
corresponding voltage.

COMPONENT SYMBOLS

Electronics engineers use a visual language that gives every
component its own symbol. Symbols are linked to show how a circuit is made.

RESISTOR COLOUR CODE CHART

Resistors are made in standard resistance values. These cannot
easily be printed on a resistor as numbers, because the resistor’s body
is too small. Coloured stripes are used instead. Common resistance values range
from 10Ω (10 ohms) to 1MΩ (a million ohms) Fourth stripe

The first two stripes on a resistor stand for numbers. The third
says how many zeros to add to these. The fourth
stripe shows the resistor’s accuracy. A gold stripe shows the
resistor’s actual value could be 5% more or 5% less than
220Ω.

TRANSISTORS

Transistors make modern electronics possible. They allow
tiny electric currents to control much bigger currents. This is called
amplification. It makes the link between a small signal that says what we want
to do and the electrical power that actually does it. The transistors inside a
radio, for example, can amplify tiny signals from the aerial to produce loud
sounds.

Transistors come in different sizes. The transistors in computer
chips are microscopic. Others may be 2.5 cm (1 in) across – big enough
to control a motor or deliver high-energy sound. Those shown here (actual size)
include a relative of the transistor called the thyristor. Between them, they
can handle everything from light bulbs to radio waves.

Current between a transistor’s emitter and collector is
normally blocked by the base. But when a small current carrying a signal flows
from the emitter to the base, lots of electrons can get through the base to
form a much bigger current from the emitter to the collector. The current is a
copy of the original signal, so the transistor amplifies it.

OPTOELECTRONICS

Optoelectronics links electronics with light. Its
simplest device is the light-dependent resistor (LDR), used in lights that turn
on by themselves at night. Light-emitting diodes (LEDs) are used for bike lamps
and other signalling jobs, such as controlling a television. DVD players depend
on the laser diode, an optoelectronic device that emits the very pure light
needed to read the disc.

LIGHT-EMITTING DIODES

A light-emitting diode (LED) is a tiny chip of material in a
plastic casing. It emits light when current flows through it. The light can be
almost any colour, depending on what the diode is made of. Most LEDs contain
the rare element gallium. When this is combined with nitrogen and another rare
element, indium, it can give blue light. With arsenic and phosphorus it can
give red light.

REMOTE CONTROL

When you change TV channels, an LED in the remote control sends
out invisible pulses of infra-red light. These are picked up by a
light-sensitive transistor, or phototransistor, at the front of the TV. The
pulses are generated by a microchip inside the control. They form a code that
tells the television what to do.

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